A plenum is the air return path of a central air-handling system, whether ductwork or the open space above a dropped ceiling.
A plenum is a space used to handle air in a building's central air-handling system, serving as part of the path that returns or distributes air. It may be enclosed ductwork or, commonly, the open space above a dropped ceiling that is used for air return. Because these spaces carry moving air throughout a building, they are treated specially with respect to fire safety.
The significance of a plenum for cable is that the moving air in such a space could spread smoke and flame rapidly in a fire. For this reason, cable installed in plenum spaces must meet strict fire-performance requirements for low flame spread and low smoke, which is why plenum-rated cable exists.
In commercial and industrial buildings, plenum spaces are common pathways for communication and other low-voltage cable. Recognizing what qualifies as a plenum is part of selecting the correctly rated cable for the location.
Because the moving air in such a space could spread smoke and flame rapidly in a fire, cable installed in a plenum must meet strict fire-performance requirements for low flame spread and low smoke, which is the reason plenum-rated cable exists as a distinct category.
At Windy City Wire, the focus on low-voltage cable includes plenum-rated products suited to air-handling spaces. Supplying cable with the appropriate fire ratings helps commercial facilities apply the correct product where plenum conditions apply.
NEC (National Electrical Code), which defines requirements for cable in plenum spaces